This image shows young and old white dwarf stars — the burned-out relics of normal stars — in the ancient globular star cluster NGC 6397.

Taken by Hubble's Advanced Camera for Surveys, it reveals young white dwarfs less than 800 million years old and older white dwarfs between 1.4 and 3.5 billion years old. Hubble researchers distinguished the younger from the older white dwarfs based on their color and brightness. The younger white dwarfs are hotter and therefore bluer and brighter than the older ones.

The astronomers were surprised to find young white dwarfs far away from the cluster's core. They had assumed that the youngsters would reside at the center and migrate over time to the cluster's outskirts. The astronomers proposed that the cluster stars that burn out as white dwarfs are given a boost that propels them to the edge of the cluster.

Please click here to visit the HubbleSite and see the complete description and identification of specific young and older white dwarfs shown in this picture.

December 4, 2007
Credit: NASA, ESA, and H. Richer (University of British Columbia)